AssistStructure improvements

Added isOpaque() for Views
Added getAcquisitionStartTime(), ...EndTime()
Added isHomeActivity()

Bug: 30895450
Test: cts-tradefed run cts-dev -m CtsAssistTestCases -t
android.assist.cts.AssistStructureTest#testAssistStructure

Change-Id: I294a089aa3454ebfebf0442036d98ccb86cb2827